1. Field of the Invention
A method is provided for improving automatic analysis of defect printability on a mask or wafer. Specifically, identification of defect versus reference images can be done quickly and accurately in a die-to-die inspection mode, thereby providing an optimized analysis.
2. Discussion of the Related Art
In high-density IC designs, those skilled in the art of integrated circuit fabrication have recognized the importance of using masks that provide accurate representations of the original design layout. Unfortunately, a “perfect” mask is not commercially viable. In fact, even under optimal manufacturing conditions, some mask defects can occur outside the controlled process.
A defect on a mask is any deviation from the design database (i.e. an irregularity) that is deemed unacceptable by an inspection tool or an inspection engineer. A mask defect printability system designed by Numerical Technologies, Inc. provides mask quality assessment without resorting to an actual exposure of a wafer. This mask defect printability system is described in U.S. patent application Ser. No. 09/814,023 (herein referenced as the NTI system), entitled, “System and Method of Providing Mask Defect Printability Analysis”, which was filed on Mar. 20, 2001 and is incorporated by reference herein.
In the NTI system, a physical mask and a corresponding, defect-free reference image are inspected. This inspection identifies any defects of the physical mask compared to the reference image. If a defect is identified, a defect area image of the defect and the area surrounding the defect from the physical mask as well as the corresponding area image from the reference image are provided to a wafer image generator. The wafer image generator generates simulations of the image data, i.e. for the physical mask and reference image.
In one embodiment, a defect printability analysis generator receives the simulated wafer images of the physical mask and the reference image from the wafer image generator. The two simulated wafer images can be aligned in a pre-processing operation. Alignment can be done using alignment keys having predetermined coordinates on the mask as well as defect free patterns on the mask. When these coordinates and patterns are aligned, the features provided on those masks (as well as on wafer images of those masks) are also aligned.
After alignment, defect analysis on the simulated wafer images can be done. In one embodiment, the defect printability analysis generator can output a defect severity score (DSS) in an impact report. This impact report can be used to reduce human error in defect printability analysis. For example, perhaps a predetermined DSS score could indicate that the printed features (as simulated by the NTI system) would have significant performance issues, but that repair of the physical mask is possible. On the other hand, perhaps a higher DSS score than above could indicate not only performance issues, but that re-fabrication of the physical mask is recommended. Thus, by providing a numerical result having an associated meaning for each number, a technician can proceed efficiently and without error to the next action, e.g. repair of the physical mask or re-fabrication of the physical mask.
Of importance, the defect-free reference image used in the defect printability analysis can be one of the following: a simulated image of the layout of the physical mask or a defect-free area of the physical mask having the same pattern. When the reference image is a layout, the simulated image can easily be identified as the reference (versus defect) image because of its source (that is, the layout is assumed to be defect-free). However, when two mask images captured from a die-to-die inspection mode are used in defect analysis, the question arises as to which image is the reference image. Unfortunately, misidentification of the reference image can adversely impact the accuracy of the defect severity score provided in the impact report. Therefore, a need arises for a quick, accurate, and automatic method of distinguishing defect and reference images.